The term "carbohydrate" embraces a wide variety of chemical compounds having the general formula (CH.sub.2 O).sub.n and encompasses such compounds as monosaccharides, disaccharides, oligosaccharides, polysaccharides and their aminated, sulfated, acetylated and other derivated forms (U.S. Pat. No. 5,288,637, this patent, as well as all other patents and publications disclosed herein are incorporated by reference). Oligosaccharides are chains composed of sugar units, which are also known as monosaccharides. Sugar units can be arranged in any order and linked by their sugar units in any number of different ways. Id. Therefore, the number of different stereoisomeric oligosaccharide chains possible is exceedingly large. Id.
Numerous classical chemical techniques for the synthesis of carbohydrates have been developed, but these techniques require selective protection and deprotection. Organic synthesis of oligosaccharides is further hampered by the lability of many glycosidic bonds, difficulties in achieving regioselective sugar coupling, and generally low synthetic yields. Therefore, unlike peptide synthesis, traditional synthetic organic chemistry cannot provide for quantitative, reliable synthesis of even fairly simple oligosaccharides.
Recent advances in oligosaccharide synthesis have occurred with the characterization, cloning and isolation of glycosyltransferases. These enzymes can be used in vitro to prepare oligosaccharides, polysaccharides and other glycoconjugates. The advantage of biosynthesis with glycosyltransferases is that the glycosidic linkages formed by enzymes are highly stereo and regiospecific. Each enzyme catalyzes the linkage of specific sugar residues to other specific acceptor moieties, such as an oligosaccharide, lipid or protein. For example, U.S. Pat. No. 5,288,637 discloses the stoichiometric synthesis of oligosaccharides using acceptor moieties, purified sugar-nucleotides and glycosyltransferases. The problem with this process, however, is that it is not commercially feasible due to the extremely high cost of the purified sugar-nucleotides. Therefore, a need exists for an economical process of synthesizing oligosaccharides using sugar-nucleotides and glycosyltransferases.